Optical mass data stores such as the "compact disc" (CD) are based on plastic deformation of a thin polymer layer, and therefore cannot at all, or only at some expense, be deleted by thermal treatment.
Optical stores based on evaluation of the phase position of linear measurement signals make use in particular of holographic methods (Seok Ho Song, Sin-Chong Park, Sang Soo Lee: "Optical implementation of quadratic associative memory by using the polarization-encoding process", Optics Letters, vol. 15 (1990) 1389-1391), which serve predominantly for storage of images and not data. Essential in holographic recording are materials having a large modulation of the refractive index (Wu, S., Chen, J., Low, P., Lin, F.: "Randomly addressable read/write/erase holographic memory based on a dyepolymer recording medium", Proceedings of the SPIE--The International Society for Optics Engineering 1662 (1992) 168-174). Photorefractive crystals with correspondingly large volume and low response times are expensive to manufacture and are associated with correspondingly high costs.
In another holographic method, linear optical polarisation is used to store binary information of an image element (in pixels). On the basis of a transition in conformation in the molecules, an optically-induced double refraction and dichroism result (Natansohn, A., Xie, S., Rochon, P.: "Azo Polymers for reversible Optical Storage, 2. Poly(4'-[[2-(acryloxyloxy)ethyl]ethylamino]-2-chloro-4-nitroazobenzene)", Macromolecules, 25 (1992), 5531-5532). With the aid of a diffuse lens system the information is inscribed and again read out by differently polarised light. However, image recognition requires the development of expensive computer programs (Seok, H. S., Song, S. L.: "Properties of holographics associative memory prepared by the polarization encoding process", Appl. Optics 27 (1988) 3149-3154.